Light-coupled, voltage-controlled constant-power source

ABSTRACT

A solid-state circuit for supplying constant power to a varying resistive load in response to a control voltage includes a light source and a photosensitive resistor. Load current is generated by a load current generator operating in response to a control signal. Resultant voltage drop across the load controls a second current generator which is serially connected with a light source. A third current generator, controlled by an independent control voltage, is serially connected with a photosensitive resistor exposed to the light source. Voltage on the photosensitive resistor comprises the control signal which controls the load current generator.

Inventors Appl. No.

Filed Patented Assignee Robert Louis McGlll, Jr.

Fallston;

Norman Green, Timonium, both 01 Md. 861,782

Sept. 29, 1969 Dec. 7, 197 1 The Bendix Corporation LIGHT-COUPLED, VOLTAGE-CONTROLLED CONSTANT-POWER SOURCE l4 Clalms, 2 Drawing Flgs.

3,383,617 5/1968 Harkenrider et a1 323/21 UX 3,431,464 3/1969 Brischnik 323/21 UX 3,458,801 7/1969 Polson..... 323/21 3,496,452 2/1970 Vollink 323/21 Primary Examiner.l. D. Miller Assistant Examiner-A. D. Pellinen Atmmeys Plante, Arens, Hartz, Smith & Thompson, Bruce L. Lamb and William G. Christoforo ABSTRACT: A solid state circuit for supplying constant power to a varying resistive load in response to a control volt- U.S. C1 323/4, age includes a light source and a photosensitive resistor. Load 323/21 current is generated by a load current generator operating in Int. Cl G05f 1/56, response to a control signal. Resultant voltage drop across the G051 l/66 load controls a second current generator which is serially con- Fleld of Search 307/117, nected with a light source. A third current generator, con- 311;315/158;317/124;323/4,21,22T trolled by an independent control voltage, is serially connected with a photosensitive resistor exposed to the light References cued source. Voltage on the photosensitive resistor comprises the UNITED STATES PATENTS control signal which controls the load current generator. 3,359,483 12/1967 Biard 323/21 O ll PATENIEU 0E0 719?: 34626276 FIG. 1.

FIG. 2.

VINVENTOFQS ROBERT L. MC GILL,JR. NORMAN GREEN AT onNE LIGHT-COUPLED, VOLTAGE-CONTROLLED CONSTANT-POWER SOURCE BACKGROUND OF THE INVENTION This invention relates to a constant electrical power circuit which maintains constant the electrical power dissipated in a varying resistance load and more particularly to such a power circuit which is constructed of solid-state elements and whose output power can be modulated by a control voltage.

The electrical power dissipated in a resistive element depends upon three variable quantities: resistance of the element; current through the element; and voltage dropacross the element. Electrical power sources arenormally provided to supply. either-a constant current or a constant voltage and wherisucha powersource is connected to deliver power to a resistive element whose resistance remains constant, the power dissipated therein will remain constant. However, if the resistance of the element varies, a constant voltage or a constant-current power source alonecannot supply unvarying power to the element. In this case, it is necessary, in order to maintain constant power dissipation in the. resistive element, to vary both the current through the resistance and the voltage thereacross.

SUMMARY OF THE INVENTION A circuit has been devised which delivers constant power to a varying resistor. This is accomplished by sensing the voltage drop across the resistor and, through the use of a novel feedback means, varying the current through the resistor in response to the voltage drop. Briefly, the voltage drop across the resistor is used to modulate a current source, with the modulated current flowing through a radiant energy source, in this particular embodiment alight-emitting diode, whose radiant output is proportional to the current flowing therethrough. A voltage modulator in the form of another current generator has its operating point set: by an externally supplied voltage, and additionally comprises a radiant energy. sensitive resistor which modulates a control voltage. This control voltage is applied to a load current source which supplies current to the varying resistor thus controlling the current therethrough.

It is thus an object ofithis invention to provide a constant- .power electrical circuit.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic of the invention. FIG. 2 shows a means for physically coupling a lightemitting diode to a responsive photosensitive resistor.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to.-FIG. 1, a resistor R; whose resistance value varies with respect to time is connected between ground and the collector electrodeof the transistor A current i, flows through R producing a resultant collector voltage with respect to ground, e, which is the voltage drop across resistor R R, is connected in the serial circuit comprised of the emitter-collector circuit of transistor 0 and resistor R between positive voltage bus 11 and ground. A current i flows" into the emitter electrode of transistor Q Voltage e, is sampled at the base electrode of transistor Q whose emitter electrode is connected to ground through R -and whose collector electrode is connected to the positive voltage bus 11 through light-emitting diode 12.0, collector current i,.excites lightemitting diode 12 to emit radiant energy which excites photosensitive-resistor R- .R is connected between the positive voltage bus I l'and thecollector electrode of transistor 0,. A resistor R, is connected between the emitter electrode of transistor Qi-and ground. Controlvoltage e,, from a source not shown, is applied to the base electrode of transistorQ Q collector voltage, e,, is sampled at the base electrode of emitter follower 0, whose collector isconnected to the positive bus 11 and whose emitter electrode is connected to the base electrode of transistor 0,.- R'esistor'R, is connected between the emitter electrode of transistor 0. and ground.

As-aforementioned; this circuit operates to maintain the following relationship: I

15 P =e j =K where'P the power dissipated through resistor R and K is a constant. To accomplish this, transistor 0,-

operates as a current generator, regulated in response to base voltage a to generate the resultant collectorv current i, which flows through resistor R) The resultant voltage drop, e across resistor R, is applied to the base electrode of transistor 0,, which also operates asa' current generator to generate in response thereto collector current i -flowing in the light emitterdiode 12.

Light emitter diode l2 is physically disposed in close proximity to'photosensitive resistor R so that the'resistance' thereof will vary in accordancewith the-light emitted by the light-emitting diode, and hence in accordance with the current flow through thisydiode. It can be seen that the'current flow i, throughresistor R assuming, as is thecase, that resistor R remains constant, is dependent on control-voltage e,. O, col-' lector voltage resulting from current i, flow through R is applied to emitter follower comprised of transistors Q, and resistor R to the base electrode of transistor 0 The voltage at the base electrode oftransistorQ has been designated as e in the figure and it should be obvious that e, is equal to 2 except for the'diode'drop across the base emitter diode of emitter follower Q... As aforementioned, e and hence e comprises the control for-current generatorQ Theamount ofilight emitted by thelight-emittingdiode 12 is dependent upon" the current which passes through it. At the same time the resistance value of-photosensitive resistor R varies inversely as the amount of light from light emitting diode 12 which impinges upon it. Hence the relationship:

where K is a coupling constant determined by the characteristics of resistor R and the light-emitting diode and the physical coupling therebetween.

Expression (1 )-can beused as a basis for showing that the power dissipated in resistor R, is a constantand a function of K the control voltage e, andothercircuitparameters. In the following mathematical-analysis of the circuit diode voltage drops are neglected'because they aresmall relative to the other voltage drops.

Using the expression l and the obvious relationship:

50 it can be seen'that:

Since diode voltage drops can be neglected e =e, so that:

so that:

K R R1Ra 5 From equation (5) it can be seen that for every distinct and constant level of control voltage e, there is a corresponding constant power dissipated in resistor R In other words, if e, is maintained constant, the power dissipated in R remains constant, or conversely, the power dissipated in R can be modul lated by varying control voltage e,. l Referring now to F IG. 2, there is seen convenient means for physically coupling light-emitting diode 12 to photosensitive l resistor R when both of these elements are packaged in a l conventional transistor can configuration. As shown, the ele-! l ments are inserted end to end into tube 14 which has an internal diameter equal to the external diameter of the elementsand is of such length that the light-emitting face 15 of light-= emitting diode 12 is maintained in close proximity to the photosensitive face 16 of photosensitive resistor R The elements are held in place by any convenient means such as by the use of tape 17. The finish of the internal surface of tube 14 is of no consequence, it only being required for constant power dissipation in the load that the coupling factor K between the two elements be constant over their intended 2 range of use. 1 Having disclosed the above preferred embodiment of our; invention, we hereby claim the subject matter including alterations and modifications thereof encompassed by the true scope and spirit of the appended claims.

The invention claimed is: 1. Means for supplying a constant power to a varying resistive element comprising: 1 means responsive to the voltage drop across said element.

for generating a control current which is directly propori tional to said voltage drop;

means responsive to said control current for generatingradiant energy;

a source of a first control voltage;

means responsive to said first control voltage and said radiant energy for generating a second control voltage; and, means responsive to said second control voltage for controlling current through said element. 1

2. Means as recited in claim 1 wherein said radiant energy I generating means comprises a light-emitting diode.

3. Means as recited in claim 2 wherein said means for generating a second control voltage comprises:

means responsive to said first control voltage for generating a second current; and, photosensitive resistive means responsive to said radiant energy and disposed to sample said second current, the voltage drop across said photosensitive resistive means? being related to said second control voltage.

4. Means as recited in claim 3 with additionally means for] mounting said light-emitting diode in a fixed, predetermined position with respect to said photosensitive resistive means. 1

5. Means as recited in claim 3 with additionally mounting means for maintaining the radiant energy coupling factor between said light-emitting diode and said photosensitive resistive means constant over the operating range of said con- 560 stant-power means.

6. A constant-power source for a resistive element comprismg: l

means responsive to the voltage across said element fori generating radiant energy; f a source of a first control voltage external to said constantpower source; means responsive to said first control voltage and said radiant energy for generating a second control voltage; and,

means responsive to said second control voltage for controlling current through said element.

7. A constant-power source as recited in claim 6 wherein said second control voltage generating means comprises:

means responsive to said first control voltage for generating a first control current;

variable resistance means responsive to said radiant energy and said first control current for generating said second control voltage.

8. A constant-power source as recited in claim 7 wherein said radiant energy generating means comprises means responsive to said voltage across said element for generating a second control current and means responsive to said second control current for generating said radiant energy.

9. A constant-power source as recited in claim 8 wherein said radiant energy generating means comprises a lightemitting diode and said variable resistance means comprises a photosensitive resistor.

10. A constant-power source for a resistive load comprising:

a load current generator responsive to a first control signal for regulating current through said load, the resultant voltage drop across said load comprising a load voltage;

means responsive to said load voltage for generating a second current;

means responsive to said second current for generating a feedback signal;

a second control signal source external to said constantpower source; and,

means responsive to said second control signal and said feedback signal for generating said first control signal.

11. A constant-power source as recited in claim 10 wherein said first control signal generating means comprises:

means responsive to said second control signal for generating a third current; and,

means responsive to said feedback signal and said third current for generating said first control signal.

12. A constant-power source as recited in claim 11 wherein said means for generating a feedback signal comprises a lightemitting diode, said feedback signal comprising radiant energy generated thereby and arranged to sample said second current; and, wherein said means for generating said first control signal comprises a photosensitive resistor responsive to said radiant energy and arranged to sample said third current, a resultant voltage drop across said photosensitive resistor comprising said first control signal.

13. A constant-power source as recited in claim 12 wherein said light-emitting diode and said photosensitive resistor are physically fixed with respect to one another.

14. A constant-power source for a resistive load comprising:

a direct current source having first and second terminals, said resistive load having one end connected to said second terminal, and a second end;

a first resistor having one end connected to said first terminal, and a second end a first transistor having a base electrode and a collectoremitter circuit for connecting said first resistor and said resistive load second ends;

a light-emitting diode having one end connected to said first terminal, and a second end;

a second resistor having one end connected to said second terminal, and a second end;

a second transistor having a base terminal connected to said resistive load second end, and a collector-emitter circuit for connecting said light-emitting diode second end with said second resistor second end;

a photosensitive resistor having one end connected to said first terminal, and a second end;

a third resistor having one end connected to said second terminal, and a second end;

a control voltage source;

a third transistor having a base electrode connected to said control voltage source, and a collector-emitter circuit for connecting said photosensitive resistor second end with said third resistor second end; and,

means connecting said photosensitive resistor second end to said first transistor base electrode.

l l l t i 

1. Means for supplying a constant power to a varying resistive element comprising: means responsive to the voltage drop across said element for generating a control current which is directly proportional to said voltage drop; means responsive to said control current for generating radiant energy; a source of a first control voltage; means responsive to said first control voltage and said radiant energy for generating a second control voltage; and, means responsive to said second control voltage for controlling current through said element.
 2. Means as recited in claim 1 wherein said radiant energy generating means comprises a light-emitting diode.
 3. Means as recited in claim 2 wherein said means for generating a second control voltage comprises: means responsive to said first control voltage for generating a second current; and, photosensitive resistive means responsive to said radiant energy and disposed to sample said second current, the voltage drop across said photosensitive resistive means being related to said second control voltage.
 4. Means as recited in claim 3 with additionally means for mounting said light-emitting diode in a fixed, predetermined position with respect to said photosensitive resistive means.
 5. Means as recited in claim 3 with additionally mounting means for maintaining the radiant energy coupling factor between said light-emittIng diode and said photosensitive resistive means constant over the operating range of said constant-power means.
 6. A constant-power source for a resistive element comprising: means responsive to the voltage across said element for generating radiant energy; a source of a first control voltage external to said constant-power source; means responsive to said first control voltage and said radiant energy for generating a second control voltage; and, means responsive to said second control voltage for controlling current through said element.
 7. A constant-power source as recited in claim 6 wherein said second control voltage generating means comprises: means responsive to said first control voltage for generating a first control current; variable resistance means responsive to said radiant energy and said first control current for generating said second control voltage.
 8. A constant-power source as recited in claim 7 wherein said radiant energy generating means comprises means responsive to said voltage across said element for generating a second control current and means responsive to said second control current for generating said radiant energy.
 9. A constant-power source as recited in claim 8 wherein said radiant energy generating means comprises a light-emitting diode and said variable resistance means comprises a photosensitive resistor.
 10. A constant-power source for a resistive load comprising: a load current generator responsive to a first control signal for regulating current through said load, the resultant voltage drop across said load comprising a load voltage; means responsive to said load voltage for generating a second current; means responsive to said second current for generating a feedback signal; a second control signal source external to said constant-power source; and, means responsive to said second control signal and said feedback signal for generating said first control signal.
 11. A constant-power source as recited in claim 10 wherein said first control signal generating means comprises: means responsive to said second control signal for generating a third current; and, means responsive to said feedback signal and said third current for generating said first control signal.
 12. A constant-power source as recited in claim 11 wherein said means for generating a feedback signal comprises a light-emitting diode, said feedback signal comprising radiant energy generated thereby and arranged to sample said second current; and, wherein said means for generating said first control signal comprises a photosensitive resistor responsive to said radiant energy and arranged to sample said third current, a resultant voltage drop across said photosensitive resistor comprising said first control signal.
 13. A constant-power source as recited in claim 12 wherein said light-emitting diode and said photosensitive resistor are physically fixed with respect to one another.
 14. A constant-power source for a resistive load comprising: a direct current source having first and second terminals, said resistive load having one end connected to said second terminal, and a second end; a first resistor having one end connected to said first terminal, and a second end a first transistor having a base electrode and a collector-emitter circuit for connecting said first resistor and said resistive load second ends; a light-emitting diode having one end connected to said first terminal, and a second end; a second resistor having one end connected to said second terminal, and a second end; a second transistor having a base terminal connected to said resistive load second end, and a collector-emitter circuit for connecting said light-emitting diode second end with said second resistor second end; a photosensitive resistor having one end connected to said first terminal, and a second end; a third resistor having one end connected to said second terminal, and a second end; a control voltage source; a third transistor having a base electrode connected to said control voltage source, and a collector-emitter circuit for connecting said photosensitive resistor second end with said third resistor second end; and, means connecting said photosensitive resistor second end to said first transistor base electrode. 